GitHub actions: use clang-12 on Linux
GitHub actions: use clang-12 on Linux

DiligentGraphics/DiligentSamples,DiligentGraphics/DiligentSamples,DiligentGraphics/DiligentSamples,DiligentGraphics/DiligentSamples,DiligentGraphics/DiligentSamples,DiligentGraphics/DiligentSamples